It is desirable, particularly in cellular base station applications, to be able to accurately and quickly control the power supply voltage provided to a radio frequency (RF) amplifier. Linear power amplifiers are fundamentally inefficient. This means that large, expensive RF power transistors are required and that these transistors require large heat sinks. This increases the size and operating cost of a cellular base station. By selecting and adjusting the power supply voltage to such an amplifier, it is possible to dramatically improve the electrical efficiency of the amplifier. This is particularly important in base station applications where the cost of the power supply and appropriate cooling apparatus makes up a significant proportion of the overall cost of the base station. Thus improvements in the area of efficiency allow smaller power supplies, reduced losses (generally released as heat in the base station) and reductions in the capacity and therefore cost of the base station cooling apparatus.
The conventional approach to this problem is to adjust to the power supply voltage to the RF amplifier synchronously with the envelope of the signal to be amplified by the RF amplifier.
This is usually achieved by pulse width modulating the output of the power supply. However, it must be noted that a four-channel UMTS system has envelope bandwidths typically in the range of 10 to 20 megahertz. To modulate a power supply at this rate using conventional pulse width modulated (PWM) power supplies would require a PWM rate somewhat in excess of these frequencies (to ensure that quantisation noise is moved out of the band of interest). Typically, such power supplies have a plurality of phases. Each phase contributes a predetermined voltage to an overall composite output voltage which is summed with that of the contribution from other phases. Straightforward pulse width modulation is unsuitable since it has insufficient noise performance. Furthermore, the efficiency of such conventional power supplies is low due to high switching losses at high switching frequencies and to high peak currents. One prior art option is a so-called “8+4” arrangement in which there are eight phases contributing an equal voltage to the output voltage, and four intermediate phases between each of the eight phases contributing a smaller amount. However, this arrangement provides a marginal noise performance and because the phases are not all equal, balancing between phases is difficult which can lead to early failure of the switching components and high switching losses.